Dynamical Phase Transitions and the Glass Transition
Glass is widespread in our society, but its physical understanding is still a very challenging open problem. While we know that for liquid water there is an equilibrium phase change as it evaporates, yet there is no consensus on whether for glasses a true equilibrium phase change occurs at low temperatures. Gaining a better understanding of glasses is complex because low temperature equilibrium glasses are hard to access, both in experiments and computer simulations.
In order to overcome these difficulties, we have used a clever numerical algorithm that combines Transition Path Sampling and Umbrella Sampling [see arXiv:1603.06892]. This allows us to drive a paradigmatic atomistic glass former off-equilibrium in a controlled manner in order to
gather normally inaccessible equilibrium thermodynamic information about its very low temperature behaviour. We prove the existence of an exceptionally low energy disordered state and highlight that it coexists with the normal liquid in a wide range of temperature. Intriguingly, we find that the two states become undistinguishable at temperatures at which the viscosity is so large that the system behaves mechanically as a solid.
These findings clarify the role of structure and dynamics in viscoelastic fluids and provide a robust link between non-equilibrium processes and equilibrium theory of the glass transition.
Interface Growth Kinetics
It is of paramount importance for materials-science to study the time evolution of different phases (liquid, amorphous or crystalline solid) for meso and macroscopic systems. This is often modeled with coarse-grained theories such as phase field, phase field crystal or, at a more fundamental level, dynamic density functional theory (DDFT). Yet, the details of the dynamics and the kinetic processes that trigger the progression of one phase at the expenses of another can be resolved in an accurate manner by the means of molecular dynamics, which unfortunately is intimately bounded to the exploration of relatively small length and short time-scales. Determining which assumptions of the coarse-grained theories can be motivated from a microscopic point of view is therefore of fundamental importance. To do so, we have focused on the non-equilibrium dynamics of liquid-solid interfaces, studying simple but yet realistic model systems of crystal growth by the means of molecular dynamics, with the perspective of a thorough comparison with the predictions of several alternative DDFT models.
Short polymers crystal nucleation and growth
Interested in the still open questions of the kinetics of polymers crystallization, I have been studying the dynamics of short flexible chains. We have analyzed the formation of the critical nucleus and the crystal growth process in a model system of eicosane. We have determined via committor analysis the characteristic nucleus size and we have shown that the chains that form the critical nucleus first align, then straighten, and finally the local crystal structure forms.
The growth of the crystal advances mainly through a sliding-in process on the lateral surface, which takes place in a correlated way, i.e. chains tend to get attached in clusters.
Large deviations and kinetically constrained models
During the PhD I have mainly worked on transport problems and their study via numerical methods.
In particular, I have considered exclusion processes and kinetically constrained models in order to explore some recent advances in modern statistical mechanics.
The study of the large deviation functions of dynamical observables applied to these models allows to show a dynamical phase transition related to the jamming and glass transition problem. Tools from the thermodynamics of histories allow to spot heterogeneities in the driven flow of kinetically constrained particles. See my PhD thesis.
Biologically inspired asymmetric exclusion processes
Biological transport processes in cells take place on substrates that are often coupled to the active motion of macromolecular complexes, such as motor proteins on microtubules or ribosomes on mRNAs. Inspired by biological processes such as protein synthesis by ribosomes and motor protein transport, we have discussed the concept of localized dynamical sites coupled to a driven lattice gas dynamics. We investigated the phenomenology of transport in the presence of dynamical defects and found a regime characterized by an intermittent current and subject to severe finite-size effects. Our results demonstrate the impact of the regulatory role of the dynamical defects in transport not only in biology but also in more general contexts.
City traffic by simple models
During my master degree, I have been working at the modelling of the traffic flow fundamental graph in a town. See my master thesis (in Italian).